The present invention relates generally to stepper motors, and more particularly to a closed loop resonance compensation circuit for stabilizing the stepper motor control system while operating in the mid-frequency resonance region.
As is generally known, a stepper motor provides controllable speed or position in response to input control pulses commonly applied from an appropriate control circuit Since the stepper motor increments in a precise amount with each control pulse it converts digital information, as represented by the input control pulses, to corresponding incremental rotation.
Conventionally there are three regions of operation in a stepper motor in terms of speed. In the low frequency region, the stepper motor develops full value or maximum torque. This region normally is in the range of about 70 to 200 pulses per second, dependent upon motor size.
The error free start-stop (EFSS) region normally falls within the range of about 120-1200 pulses per second, dependent upon motor size. In this EFSS region, the stepper motor can be started or stopped at any point without error.
The slew region is a relatively high speed region which generally is associated with acceleration and deceleration controls. Typically, the stepper motor is accelerated or ramped up to the slew rate and later is decelerated or ramped down from the slew rate to stop at the desired end position for the motor. In order to maintain synchronism between the input control pulses and the motor speed or position, it is common to accelerate the motor slowly from the error free start stop rate to the slew rate, in a manner provided by the motor control circuit.
The slew curves for a stepper motor show the maximum speeds and torques at which the motor may be operated without losing steps, provided that acceleration and deceleration control is employed. In many cases these slew curves display torque "dips" which illustrate mid-frequency resonance regions in which the motor cannot be operated for any length of time, unless suitable provision is made to stabilize the stepper motor control system while operating in the mid-frequency resonance region.
Mid-frequency resonance is a phenomena characterized by the premature loss of torque exhibited by a step motor when operated at a step rate higher than its maximum error free start-stop rate. This phenomena is analogous to the "hunting" of an unloaded synchronous motor rotor as it seeks a stable equilibrium with the rotating stator field. In an uncompensated step motor system this velocity oscillation about the velocity of the stator field is induced by a perturbation in the system, which may be caused by load changes, power supply variations, or abrupt stator velocity changes (acceleration). Once the disturbance has been introduced to the system, the rotor velocity will begin to oscillate at a low frequency (as compared to the step rate). The velocity fluctuations will increase in amplitude slowly until the rotor position leads or lags the stator field by 2 or more steps, at which time the rotor will be equally attracted to two stator poles, lose synchronism and stall.
From the description of the mechanics of mid-frequency resonance above, it may be understood that the feedback of velocity information into the step motor control system, in such a manner as to reduce the amplitude of the rotor velocity fluctuations with respect to the stator field, may greatly reduce, if not entirely eliminate, the premature loss of torque exhibited by a step motor when operated in the midfrequency resonance region.
It is therefore one object of the present invention to provide an improved method and means for stabilizing a stepper motor/control system while operating in the midfrequency resonance region. It is another object of the present invention to provide an improved stepper motor stabilization method and means which corrects for mid-frequency resonance errors by deriving a velocity error signal from the total current in the motor windings and using this signal, through feedback to the source of clock pulses, to correct the short term rotor velocity with respect to the stator field, without affecting the average velocity, and the distance traveled as determined by the command pulse train.